1. Field of the Invention
The present invention relates to a technique for receiving an Orthogonal Frequency Division Multiplexing (OFDM) signal transmitted by an OFDM system and demodulating it adequately.
2. Description of the Related Art
As a system for transmitting data, proposed in recent years is a transmission system called an Orthogonal Frequency Division Multiplexing (OFDM) system. The OFDM system transmit data by allocating data to plural carriers (i.e., sub-carriers), respectively, which are perpendicular on the frequency axis. A modulation and a demodulation commonly use an inverse fast Fourier transform (IFFT) and a fast Fourier transform (FFT), respectively. The FFT is applied in a width of a time window (i.e., an FFT window). Since frequency usage efficiency is high at the OFDM system, an application to a digital terrestrial broadcasting is widely considered and adopted by the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T), that is, the terrestrial digital broadcasting of Japan.
In the conventional digital modulation system using a single carrier, modulation of a signal is difficult under a condition (i.e., a multi-path environment) where a multi-path occurs (i.e., a reception side receiving the same radio wave by way of a plurality of paths as a result of the radio wave transmitted from the base station or broadcasting station being reflected by obstacle(s) such as building(s) in a mobile phone service or a television broadcast), because a symbol period becomes shorter with a speed of transmission. Accordingly, the OFDM system employs a multi-carrier transmission system which transmits by dividing/allocating information into a plurality of carriers, in lieu of using a single carrier. The transmission of divided data by a plurality of carriers makes it possible to make a symbol length long for each of the carriers, thereby enabling a response to a multi-path with a long delay. And it is possible to use different modulation system for each carrier because data are respectively allocated to plural carriers.
FIG. 4 shows a format of an OFDM signal. Referring to FIG. 4, carriers (i.e., sub-carriers) modulated in a time/frequency domain are indicated by circles. Solid black circles among those circles indicate carriers which are placed with Scattered Pilot (SP) information (which is called as “SP carrier” hereinafter), while white circles indicate carriers placed with data. As shown in FIG. 4, the SP information is placed one in twelve carriers in the frequency axis direction, while one in four symbols in the time axis direction.
The OFDM system is reinforced against a multi-path by adding the end part of a symbol to the beginning thereof. The added signal is called a guard interval.
For a radio wave received in a multi-path environment, a radio wave of the maximum power is called a principal wave, one received with a delay from the principal wave is called a delayed wave, and one received in advance of the principal wave is called an advanced wave. The principal wave is a radio wave usually received directly, that is, one free of a reflection, et cetera, by an obstacle. The delayed wave is also generated by a diffraction or scatter in addition to a reflection by an obstacle. Radio waves causing a hindrance to a demodulation are generally called an interference wave.
FIGS. 1A and 1B show an effect of a guard interval. These diagrams show relationship of symbols between the principal wave and delayed wave in the cases of the guard interval being present, i.e., FIG. 1A, and of the one being not present, i.e., FIG. 1B. Note that “n−1”, “n” and “n+1” respectively noted in FIGS. 1A and 1B indicate the sequence of the symbols being transmitted. This description method is the same hereinafter.
An FFT is performed in the range specified by an FFT window as the target. Because of it, the symbol n−1 of a delayed wave is imported when applying the FFT to the symbol n of a principal wave if there is no guard interval as shown in FIG. 1A, thus resulting in generating an interference between symbols in the front and back. Comparably, if there is a guard interval, a demodulation is possible without generating such interference between symbols because a symbol n−1 positioned immediately in front is not imported as shown in FIG. 1B.
A position of the FFT window is controlled by matching with the position of a symbol which is to be the target in order to perform a demodulation appropriately. The position of the FFT window is matched with the position of a symbol of the principal wave by the aforementioned control (i.e., the FFT window control) under the condition of a delayed wave in existence. The reason is that a guard interval is added in front of a symbol. Under the condition of an advanced wave in existence in place of a delayed wave, however, the next symbol of the advanced wave overlaps in the range of a symbol of the principal wave existing (refer to FIG. 2A). Consequently, if the position of the FFT window is matched with the position of a symbol of the principal wave, a demodulation becomes impossible as a result of generating interference between the symbols. Because of this, under the condition of an advanced wave in existence, an FFT window control is carried out as follows, which is now specifically described by referring to FIGS. 2A and 2B.
FIGS. 2A and 2B show an FFT window control performed in the case of an advanced wave in existence, with FIG. 2A showing the case of controlling the FFT window incorrectly and FIG. 2B showing the case of controlling it correctly.
As shown in FIG. 2A, if the position of the FFT window is matched with the symbol n of the principal wave in the same manner as in the case of the delayed wave, the symbol n+1 of the advanced wave is imported when performing an FFT of the symbol n, the correct demodulation cannot be carried out as a result of generating interference between the symbols. Comparably, if the position of the FFT window is matched with the symbol n of the advanced wave, the range of the FFT window is now the data of the symbol n for both the principal wave and advanced wave as shown in FIG. 2B, and therefore it is possible to demodulate correctly.
It is possible to demodulate correctly in a multi-path environment under which an FFT window is within a guard interval length by performing an FFT window control as described above. Therefore, a Single Frequency Network (SFN) broadcast is enabled. The SFN broadcast carries out a simultaneous broadcast at a plurality of stations by relaying and transmitting the same program on the same frequency (i.e., channel). This creates a multi-path environment; the adoption of the OFDM system accomplishes it, however.
FIG. 3 shows a configuration of a conventional OFDM receiver. A specific description at this point is of the conventional OFDM receiver by referring to FIG. 3.
The OFDM receiver, premising a digital terrestrial broadcasting for example, comprises an antenna 31, a tuner 32, an analog-to-digital (A/D) converter 33, an orthogonal demodulation unit 34, an FFT unit 35, a transmission path equalization unit 36, an error correction unit 37, an IFFT unit 38 and a delay information extraction unit 39.
The next description is of an operation thereof.
An OFDM signal received at the antenna 31 is input to the tuner 32, and only the OFDM signal selected by the tuner 32 is input to the A/D converter 33 to be digitized. The digitized OFDM signal is input to the orthogonal demodulation unit 34 to be applied by an orthogonal demodulation, thereby generating a baseband OFDM signal. The FFT unit 35 receives an input of the OFDM signal, performs an FFT by extracting a part specified by the FFT window and converts a time domain signal into a signal of a frequency domain. The transmission path equalization unit 36 estimates a transmission path and removes the influences of a noise and of a waveform distortion from the OFDM signal converted into a frequency domain. The error correction unit 37 performs an error correction of a post-compensation OFDM signal as s target after the aforementioned removal. This correction is carried out by using an error correction sign added to the OFDM signal. The post-correction OFDM signal is output as the final demodulation signal.
The IFFT unit 38 performs an IFFT by extracting a part worth an SP carrier from within the OFDM signal output from the FFT unit 35 and reverts it back to a time domain signal. The delay information extraction unit 39 detects a delay time of a transmission path (i.e., delay information) from the time domain signal of an SP carrier and performs an FFT window control by setting the position of the FFT window according to the delay time. The FFT unit 35 performs an FFT according to the position of the FFT window set by the window control.
As described above, the addition of a guard interval makes it possible to correspond to a delayed wave or advanced wave within the length of the guard interval. In the above noted SFN broadcast, there is a region where an interference wave exceeds the allowable range of the guard interval. In such a region, a generated interference causes to fail an appropriate demodulation.
Depending on a modulation system adopted by the OFDM system, it is possible to receive even if there is a certain degree of degradation by adequately matching the position of the FFT window. This makes a right matching of the position of the FFT window very important also in a multi-path environment where a multi-path exceeds the allowable range of the guard interval.
The conventional OFDM receiver shown by FIG. 3 detects delay information of a transmission path from an SP carrier and performs an FFT window control. The SP carrier is placed within an OFDM signal as shown in FIG. 4. By this, the SP carrier is usable in a three-carrier interval on the time axis and therefore a detectable range of the delay information is up to one third (⅓) symbol length. One symbol length in the case of the mode 3 of the ISDB-T is 1.008 milliseconds. Therefore the range is up to 336 (=1000/3) microsec. Considering both of the delay and advanced waves, it is up to ±⅙ symbol length. In the case of ISDB-T, mode 3, the limit is ±168 microsec.
From the above noted considerations, assuming there is a 200-microsec advanced wave, the detection limit is ±168 microsec for an FFT window control by the SP carrier, and therefore the 200-microsec advanced wave results in being regarded as a 136-microsec delayed wave in terms of its characteristic.
In the case of the mode 3 with a guard interval length being ⅛ symbol, which is common for the ISDB-T, the guard interval length is 126 microsec. The conventional FFT window control by the SP carrier is capable of performing it correctly in a multi-path environment within the guard interval length. As described above, however, such a conventional FFT window control can actually hardly perform correctly in a multi-path environment exceeding the guard interval length. Therefore, it is conceivably very important to have a capability of carrying out an FFT window control also in an environment containing an interference wave (i.e., a delayed wave or advanced wave) exceeding a range detectable by the SP carrier.
Reference technical documents include Laid-Open Japanese Patent Application Publication Nos. 2001-345775, 2003-110519, 2004-228853 and 2004-336279.